The invention relates to a pigmented thermoplastic material, a method for its production as well as its use for production of transparent shaped bodies.
With the traditional methods for production of pigmented thermoplastic material, pigment in form of powder is usually employed. This entails process-technical drawbacks, since it is necessary to add, for dispersion of powdery pigments in the thermoplastic material, dispersing adjuvants, such as waxes, oils or stearates. The addition of such adjuvants may lead to processing problems as well as loss of quality in the end product. Also, by adding these adjuvants, it is not possible to definitely assure that optimum dispersion will be achieved. Another drawback related to the use of powder pigments for production of highly pigmented pigment master batches is their low bulk-density during direct processing in the extruder.
From U.S. Pat. No. 4,474,473 and U.S. Pat. No. 6,273,599 continuous flushing methods are known of pigments in which aqueous pigment press cakes are converted to a hydrophobic organic phase. The thus developing free-flowing pigment dispersions are, however, only suitable for application in printing inks and paints.
The aforementioned problems in regard to the addition of dispersion adjuvants are solved, at least partially, by a method described in DE 103 35 863 A1. By using a specific extrusion method, it is possible to reduce the amount of dispersion adjuvants. To that end, it is suggested in DE 103 35 863 A1 that a pigment master batch is produced in that a thermoplastic polymer in the form of granulate or powder is continuously dosed into a twin-screw extruder, whereby the dosed-in polymer is melted in the extruder.
In addition, a pumpable pigment press-cake which contains 5 to 35% of pigment by weight, water and/or organic solvent, is continuously dosed under increased pressure into the melted polymer in the extruder. The pressure is hereby so high that the boiling temperature of the water and/or of the organic solvent is higher than the internal temperature of the extruder in the area of the inlet aperture of the extruder. The pigment is dispersed in the extruder from the press-cake into the polymer melt by the effect of shearing forces. The water and/or the organic solvents are removed under increased pressure through an outlet aperture of the extruder, whereby the pressure is so high that the boiling temperature of the water and/or of the organic solvent is higher than the internal temperature of the extruder in the area of the outlet aperture. Finally, the pigmented polymer melt is discharged from the extruder, cooled down and granulated.
This known method involves a continuous method, in which it matters that the pigment press cake and the water and/or the organic solvent are introduced under increased pressure into the extruder or discharged from the extruder. It permits the break-up of larger pigment agglomerations in the extruder by means of shearing force effect. However, the shearing forces in the extruder are far from being forceful enough to adequately comminute all agglomerates or coarse-grained pigment particles. Even smallest portions of coarse-grained pigment particles or agglomerates in the production of spinning fibers reduce the stability of the threads, which may result in their breaking off and, in the extreme case, even in clogging of the fine nozzles. In case of foils, the coarse-grained pigment particles lead to so-called “oversize particles” (ripening) which, in turn, entail color imperfections (pin holes) of thin-film foils. For that reason, this method cannot be used to produce flushes which are suitable for the production of spinning fibers and thin-film foils.